Abstract

Base- and precious-metal mineral deposits are located along and adjacent to Tertiary detachment faults in western Arizona and southeastern California. Mineralization is characterized by early copper and iron sulfides, followed by massive specular hematite, in turn followed by fracture-filling chrysocolla and malachite. Mineralization occurred during faulting and was associated with unusually high geothermal gradients that were at least partially due to detachment-fault movement that brought hot, lower-plate rocks up from great depth and juxtaposed them with cool, near-surface, upper-plate rocks. Mixing of ascending, reduced, metal-bearing aqueous solutions with higher level, oxidized fluids is proposed as an important process leading to oxide mineralization. Movement of the redox interface down normal-fault ramps at the same rate as displacement of hanging-wall rocks resulted in continuous mineralization at some upper-plate sites over significant periods of geologic time; this produced the largest orebodies. Aqueous solutions that precipitated iron oxides near the detachment fault carried manganese oxides to higher levels where vein and strata-bound sedimentary manganese oxides were deposited.